In Our Skies: The unfixed and fixed stars

Ancient sky-watchers in Greece – and presumably elsewhere as well – divided the stars in the nighttime sky into two broad categories: the fixed stars which, as this term implies, remained in the same locations with respect to each other, and the wandering stars, which shifted locations among the fixed stars on a regular basis.

Today we know that the wandering stars are in fact the planets within our solar system, whereas the fixed stars are true stars, at least somewhat similar to our sun but much, much farther away.

For many centuries it was believed that the fixed stars were indeed truly fixed and unmoving. However, in the early 18th century the British astronomer Edmond Halley noticed that the bright stars Sirius, Aldebaran and Arcturus – the last of which is currently visible in our northwestern skies after dusk – were approximately half a degree – the apparent diameter of the full moon – away from the positions within which they had been catalogued by the Greek astronomer Hipparchus almost 2000 years earlier.

Over the ensuing decades astronomers determined that many, if not almost all, stars are moving as well. Thus was born the concept of proper motion, i.e., the observed motion that a star exhibits across our line of sight.

Today, partially because of certain dedicated spacecraft missions, we have determined the proper motions for millions of stars. The star with the largest known proper motion is an object known as Barnard’s Star, discovered by the American astronomer Edward Barnard in 1916, which is a small red dwarf star in the constellation Ophiuchus – now in our western sky after dusk – and which is the second-closest known star to our solar system after the stars of the Alpha Centauri system.

Barnard’s Star, which can be detected with a pair of binoculars, travels against the background stars at a rate of slightly over 10 arcseconds per year, which translates into one degree – twice the apparent diameter of the full moon – every 350 years.

A star may not only travel across our line of sight, but also towards us or away from us. Its radial motion can be detected by examining its spectrum: if it is approaching us, the features of its spectrum will shift towards the high-frequency, or blue, part of the spectrum, whereas if it is moving away from us, those features will shift towards the low-frequency, or red, part of the spectrum. This is the so-called Doppler Effect, discovered by the Austrian physicist Christian Doppler in the mid-19th century, and explains why an ambulance siren increases in pitch as it approaches us and drops in pitch when it travels away from us.

By combining a star’s proper motion with its radial motion, we can determine a star’s true space motion – although we need to keep in mind that our sun’s own motion through space is reflected in our measurements. To subtract that out, we need to determine the space motions for a large number of stars; when we do so, we can then determine just what is going on with the stars in our part of the galaxy.

We can also determine what our sun is doing. It turns out that we are headed in the general direction of the constellation Hercules, roughly 10 degrees south of the bright star Vega, now high in our northwestern sky during the evening hours. This is the so-called solar apex. The exact opposite location in the sky – in the constellation Canis Major, some 13 degrees south of the bright star Sirius, now low in our southeastern sky before dawn – is the solar antapex, the direction we are coming from.

By the same token, we can determine what some of the other nearby stars are doing. The nearest star, Alpha Centauri, will come closest to our solar system in about 27,000 years, when it will be located 3.3 light-years from us instead of the present 4.4 light-years. Barnard’s Star, presently 6.0 light-years away, will pass its closest in a little less than 10,000 years, at a distance of 3.8 light-years.

Other stars have passed, or will pass, even closer. Just five years ago a German astronomer, Ralf-Dieter Scholz, discovered a very tiny star located some 20 light-years away, in the constellation Monoceros relatively close to Sirius.

Astronomers have since determined that this object – named Scholz’s Star – passed only 0.8 light-years from the sun 70,000 years ago. This is within the outer fringes of the Oort Cloud – the spherical cloud of comets that encircles the solar system – and any comets kicked up by that encounter will be making their way into the inner solar system in about two million years.

An even closer encounter involves the star Gliese 710, which is about half the size of our sun and which is located in the constellation Serpens, not too far from Barnard’s Star. Although now located some 64 light-years away from us and requiring a small backyard telescope to be seen, astronomers have found that it is headed almost directly toward us: according to the most recent determinations, in 1.3 million years it will pass less than a quarter of a light-year from the sun.

At that time, it will appear brighter than any planet in the nighttime sky except Venus and will be traveling at an apparent rate of one degree every 60 years.

The star’s miss distance is well within the Oort Cloud, and it will certainly be kicking in swarms of comets into the inner solar system. Meanwhile, if Gliese 710 possesses something akin to its own Oort Cloud, then quite a few of those objects should be passing through the inner solar system as well. The impact threat for Earth is obvious, but if humanity should manage to survive that long, hopefully our descendants of that far-future era will be able to deal with this issue when the outside galaxy comes calling.

Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere.